1. Field of the Invention
This invention relates to a ferroelectric liquid crystal material. More particularly it relates to a ferroelectric smectic liquid crystal material comprising a novel smectic liquid crystal compound and an optically active compound suitable for high-speed response liquid crystal display devices.
2. Description of the Related Art
At present, the TN (Twisted Nematic) display mode has been broadly employed for liquid crystal display elements. This TN display mode has a number of advantages such as low driving voltage, small power consumption, etc., but it is still inferior in the aspect of the response rate to emissive display elements such as cathode ray tubes, electroluminescence displays, plasma displays, etc. A novel TN display element having a twist angle elevated to 180.degree. to 270.degree. has also been developed, but its response rate is still inferior. Efforts for various improvements have been made as described above, but a TN mode display element having a high response rate has not yet been realized. However, in the case of a novel display mode using a ferroelectric liquid crystal the research of which mode has recently been extensively made, there is a possibility of notable improvement in the response rate (see Clark et al, Applied Phys. Lett., 36, 899 (1980)). This mode utilizes chiral smectic phases exhibiting ferroelectric properties such as chiral smectic C phase (hereinafter abbreviated to SC*). It has been known that phases exhibiting ferroelectric properties are not only limited to SC* phase, but also include phases of chiral smectic F, G, H, I, etc.
Various specific features have been required for ferroelectric liquid crystal materials used for ferroelectric liquid crystal display elements practically used, but at present, there is no single compound which satisfies such requirements; hence it is necessary to use ferroelectric liquid crystal compositions obtained by blending some liquid crystal compounds and/or non-liquid-crystalline compounds.
Further, not only have ferroelectric liquid crystal compositions composed only of ferroelectric liquid crystal compounds been reported, but also it has been reported in Japanese patent application laid-open No. Sho 61-195187/1986 that when one or more kinds of compounds exhibiting ferroelectric liquid crystal phases are blended with compounds or compositions exhibiting achiral smectic C, F, G, H, I phase or the like (hereinafter abbreviated to SC phase or the like) as base substances, the resulting blend can constitute a ferroelectric liquid crystal composition as a whole.
Further, it has also been reported that using a compound or a composition containing the compound each exhibiting S.sub.c phase or the like as a base substance, at least one compound which is optically active but exhibits no ferroelectric liquid crystal phase is mixed with the above compound or composition to constitute a ferroelectric liquid crystal composition as a whole (see Mol. Cryst. Liq. Cryst. 89, 327 (1982)).
In a brief summary of these facts, it is seen that when one or more kinds of optically active compounds are blended with base substances, the resulting blend can constitute a ferroelectric liquid crystal composition irrespective of whether or not the optically active compounds exhibit ferroelectric liquid crystal phases.
A smectric liquid crystal mixture which comprises a base substance exhibiting at least one achiral smectic phase and also has at least one achiral smectic phase will hereinafter be referred to as "smectic base mixture". Such a smectic base mixture is practically preferred to be a liquid crystal mixture exhibiting an SC phase over a broad temperature range including room temperature. As the component of the smectic base mixture, some compounds selected from among liquid crystal compounds of phenylbenzoates, Schiff's bases, biphenyls, phenyl-pyridines, 5-alkyl-2-(4-alkoxyphenyl)pyridines, etc. have been used.
For example, Japanese patent application laid-open No. Sho 61-291679/1986 and the international publication WO 86/06401 disclose a ferroelectric liquid crystal having a 5-alkyl-2-(4-alkoxyphenyl)pyrimidine blended with an optically active compound, and also the former reference discloses that when a ferroelectric smectic liquid crystal material comprising the above pyrimidine derivative as a smectic base mixture is used, it is possible to shorten the response time of light-switching elements. Further the former reference also discloses that a ferroelectric liquid crystal material comprising a 5-alkyl-2-(4'-alkylbiphenylyl-4)pyrimidine, the above 5-alkyl-2-(4-alkoxyphenyl)pyrimidine and an optically active compound is effective for improving the response properties.
However, as compared with other type display elements such as emissive mode displays, a further improvement in the response properties has been required for the liquid crystal display.